1. Field of the Invention
The present invention relates to a camera that engages in a focusing operation through the contrast method by using an image-capturing signal provided by an image-capturing element.
2. Description of the Related Art
The AF methods adopted in cameras in the related art include the so-called contrast method. In the contrast method, an image of a subject is captured by an image-capturing element such as a CCD and the focussed position is determined by using the image-capturing signal within the focus area. The image-capturing signal within the area are filtered through a band pass filter (BPF) and the components in the image-capturing signal that correspond to a predetermined spatial frequency band are extracted. Then, by integrating the absolute values of the components within the area, a focus evaluation value to be used for a focusing operation is obtained. This focus evaluation value represents the contrast level, and the contrast is at its highest at the point at which the focus evaluation value peaks.
This peak position matches the focussed position. The peak is ascertained by performing an operation which is known as a so-called hill-climbing (or scaling) focusing operation in the related art.
Among cameras adopting this hill-climbing AF, there are cameras that automatically set photographing conditions suited to photograph a main subject that the photographer wishes to photograph. Such photographing conditions may constitute, for instance, a distant view mode or a portrait mode.
During the hill-climbing focusing operation in the related art, focus evaluation values are calculated in a uniform manner regardless of the lens position, i.e., regardless of whether the focusing lens of the photographic lens is set toward the close-up side or the infinity side. Thus arises the following problem. For instance, let us consider a situation in which the photographing condition is set to the distant view mode, a bridge is present further frontward relative to a range of mountains in the distance and a part of the bridge is included within a focus detection area. If the focus evaluation value characteristics obtained when the focusing lens is moved from the close-up side toward the infinity side manifest a peak corresponding to the mountain range on the infinity side and also a peak corresponding to the bridge on the close-up side under these circumstances, the focal adjustment of the focusing lens is performed so as to focus on the subject with a higher peak. As a result, the focal adjustment maybe executed on the bridge although the camera is set in the distant view mode.
In addition, in a camera that performs the hill-climbing focusing operation, a focus detection area may be set over an off-axis peripheral range as well as one set on the optical axis. In this specification, such a camera is referred to as a multipoint autofocus camera and its focus detection areas are referred to as multipoint autofocus areas.
The photographic lens utilized in a multipoint autofocus camera manifests off-axis MTF characteristics that are different from the MTF characteristics manifesting on the optical axis. This results in the following problem. When a focus detection is performed by using, for instance, a reference pattern constituted of a uniform line and space pattern, focus detection signals obtained by filtering the images in the individual focus detection areas through a band pass filter should indicate levels equal to one another if the photographic lens manifests identical MTF characteristics for all the autofocus areas. However, different focus detection signals are generated in reality due to the varying MTF characteristics. For this reason, the focus evaluation values calculated based upon the focus detection signals, too, are affected by the MTF characteristics in the multipoint autofocus camera, which, in turn, significantly lowers the focal adjustment accuracy.